The properties of optical fiber make it an ideal medium for carrying signals in an ever-increasing number of applications. These properties include high bandwidth, i.e. signal carrying capacity, immunity to interference, low loss, light weight and low bulk. Optical fiber is often used in conjunction with electrical leads for carrying power, in applications such as underwater sensor arrays and remotely operated vehicles for pipeline inspection and repair. In such applications the composite cable is winched from the surface vessel. This requires a slip ring or rotary joint assembly at the winch, to permit the payout or retraction of cable while allowing continuous operation of the underwater system.
Electrical slip rings for the copper wires carrying power are well established, but fiber optic rotary joints are much more recent developments and have not been perfected to an adequate state for many applications. In some systems, wherein optical fiber has been used for signal telemetry, it was necessary to convert the optical signals to electrical signals within the winch drum, thereby losing some of the benefits of fiber optics through exposure to interference as well as introducing failure-prone and expensive components.
The use of fiber optic rotary joints has been limited by their performance. The few fiber optic rotary joints that have been reported have losses of typically 3 dB for insertion loss and -20 to -25 dB for return loss. These figures, which are for multimode fiber, are inadequate for some applications; more importantly, they are unacceptable for use with laser based singlemode fiber systems. Singlemode fiber of the type used in long-haul telecommunications is increasingly chosen for short-haul applications such as those given above, to capitalize on its higher bandwidth, lower loss and other advantages.
Oil-filling has been mentioned in other disclosures (U.S. Pat. No. 4,725,116) but not in the context of reducing return loss to permit singlemode operation.
The invention described herein gives several improvements in the state of the art of fiber optic rotary joint technology, accommodating many applications which were previously not satisfactorily addressed.